1NETLINK(7) Linux Programmer's Manual NETLINK(7)
2
6 netlink - communication between kernel and user space (AF_NETLINK)
7
9 #include <asm/types.h>
10 #include <sys/socket.h>
11 #include <linux/netlink.h>
12 netlink_socket = socket(AF_NETLINK, socket_type, netlink_family);
14
16 Netlink is used to transfer information between the kernel and user-
17 space processes. It consists of a standard sockets-based interface for
18 user space processes and an internal kernel API for kernel modules.
19 The internal kernel interface is not documented in this manual page.
20 There is also an obsolete netlink interface via netlink character de‐
21 vices; this interface is not documented here and is provided only for
22 backward compatibility.
23 Netlink is a datagram-oriented service. Both SOCK_RAW and SOCK_DGRAM
25 are valid values for socket_type. However, the netlink protocol does
26 not distinguish between datagram and raw sockets.
27 netlink_family selects the kernel module or netlink group to communi‐
29 cate with. The currently assigned netlink families are:
30 NETLINK_ROUTE
32 Receives routing and link updates and may be used to modify the
33 routing tables (both IPv4 and IPv6), IP addresses, link parame‐
34 ters, neighbor setups, queueing disciplines, traffic classes,
35 and packet classifiers (see rtnetlink(7)).
36 NETLINK_W1 (Linux 2.6.13 to 2.16.17)
38 Messages from 1-wire subsystem.
39 NETLINK_USERSOCK
41 Reserved for user-mode socket protocols.
42 NETLINK_FIREWALL (up to and including Linux 3.4)
44 Transport IPv4 packets from netfilter to user space. Used by
45 ip_queue kernel module. After a long period of being declared
46 obsolete (in favor of the more advanced nfnetlink_queue fea‐
47 ture), NETLINK_FIREWALL was removed in Linux 3.5.
48 NETLINK_SOCK_DIAG (since Linux 3.3)
50 Query information about sockets of various protocol families
51 from the kernel (see sock_diag(7)).
52 NETLINK_INET_DIAG (since Linux 2.6.14)
54 An obsolete synonym for NETLINK_SOCK_DIAG.
55 NETLINK_NFLOG (up to and including Linux 3.16)
57 Netfilter/iptables ULOG.
58 NETLINK_XFRM
60 IPsec.
61 NETLINK_SELINUX (since Linux 2.6.4)
63 SELinux event notifications.
64 NETLINK_ISCSI (since Linux 2.6.15)
66 Open-iSCSI.
67 NETLINK_AUDIT (since Linux 2.6.6)
69 Auditing.
70 NETLINK_FIB_LOOKUP (since Linux 2.6.13)
72 Access to FIB lookup from user space.
73 NETLINK_CONNECTOR (since Linux 2.6.14)
75 Kernel connector. See Documentation/driver-api/connector.rst
76 (or /Documentation/connector/connector.* in kernel 5.2 and ear‐
77 lier) in the Linux kernel source tree for further information.
78 NETLINK_NETFILTER (since Linux 2.6.14)
80 Netfilter subsystem.
81 NETLINK_SCSITRANSPORT (since Linux 2.6.19)
83 SCSI Transports.
84 NETLINK_RDMA (since Linux 3.0)
86 Infiniband RDMA.
87 NETLINK_IP6_FW (up to and including Linux 3.4)
89 Transport IPv6 packets from netfilter to user space. Used by
90 ip6_queue kernel module.
91 NETLINK_DNRTMSG
93 DECnet routing messages.
94 NETLINK_KOBJECT_UEVENT (since Linux 2.6.10)
96 Kernel messages to user space.
97 NETLINK_GENERIC (since Linux 2.6.15)
99 Generic netlink family for simplified netlink usage.
100 NETLINK_CRYPTO (since Linux 3.2)
102 Netlink interface to request information about ciphers regis‐
103 tered with the kernel crypto API as well as allow configuration
104 of the kernel crypto API.
105 Netlink messages consist of a byte stream with one or multiple nlmsghdr
107 headers and associated payload. The byte stream should be accessed
108 only with the standard NLMSG_* macros. See netlink(3) for further in‐
109 formation.
110 In multipart messages (multiple nlmsghdr headers with associated pay‐
112 load in one byte stream) the first and all following headers have the
113 NLM_F_MULTI flag set, except for the last header which has the type
114 NLMSG_DONE.
115 After each nlmsghdr the payload follows.
117 struct nlmsghdr {
119 __u32 nlmsg_len; /* Length of message including header */
120 __u16 nlmsg_type; /* Type of message content */
121 __u16 nlmsg_flags; /* Additional flags */
122 __u32 nlmsg_seq; /* Sequence number */
123 __u32 nlmsg_pid; /* Sender port ID */
124 };
125 nlmsg_type can be one of the standard message types: NLMSG_NOOP message
127 is to be ignored, NLMSG_ERROR message signals an error and the payload
128 contains an nlmsgerr structure, NLMSG_DONE message terminates a multi‐
129 part message. Error messages get the original request appened, unless
130 the user requests to cap the error message, and get extra error data if
131 requested.
132 struct nlmsgerr {
134 int error; /* Negative errno or 0 for acknowledgements */
135 struct nlmsghdr msg; /* Message header that caused the error */
136 /*
137 * followed by the message contents unless NETLINK_CAP_ACK was set
138 * or the ACK indicates success (error == 0).
139 * For example Generic Netlink message with attributes.
140 * message length is aligned with NLMSG_ALIGN()
141 */
142 /*
143 * followed by TLVs defined in enum nlmsgerr_attrs
144 * if NETLINK_EXT_ACK was set
145 */
146 };
147 A netlink family usually specifies more message types, see the appro‐
149 priate manual pages for that, for example, rtnetlink(7) for
150 NETLINK_ROUTE.
151 Standard flag bits in nlmsg_flags
153 ────────────────────────────────────────────────────────────────────────
154 NLM_F_REQUEST Must be set on all request messages.
155 NLM_F_MULTI The message is part of a multipart message
156 terminated by NLMSG_DONE.
157 NLM_F_ACK Request for an acknowledgement on success.
158 NLM_F_ECHO Echo this request.
159 Additional flag bits for GET requests
161 ────────────────────────────────────────────────────────────────────────
162 NLM_F_ROOT Return the complete table instead of a single
163 entry.
164 NLM_F_MATCH Return all entries matching criteria passed
165 in message content. Not implemented yet.
166 NLM_F_ATOMIC Return an atomic snapshot of the table.
167 NLM_F_DUMP Convenience macro; equivalent to
168 (NLM_F_ROOT|NLM_F_MATCH).
169 Note that NLM_F_ATOMIC requires the CAP_NET_ADMIN capability or an ef‐
171 fective UID of 0.
172 Additional flag bits for NEW requests
174 ────────────────────────────────────────────────────────────────────────
175 NLM_F_REPLACE Replace existing matching object.
176 NLM_F_EXCL Don't replace if the object already exists.
177 NLM_F_CREATE Create object if it doesn't already exist.
178 NLM_F_APPEND Add to the end of the object list.
179 nlmsg_seq and nlmsg_pid are used to track messages. nlmsg_pid shows
181 the origin of the message. Note that there isn't a 1:1 relationship
182 between nlmsg_pid and the PID of the process if the message originated
183 from a netlink socket. See the ADDRESS FORMATS section for further in‐
184 formation.
185 Both nlmsg_seq and nlmsg_pid are opaque to netlink core.
187 Netlink is not a reliable protocol. It tries its best to deliver a
189 message to its destination(s), but may drop messages when an out-of-
190 memory condition or other error occurs. For reliable transfer the
191 sender can request an acknowledgement from the receiver by setting the
192 NLM_F_ACK flag. An acknowledgement is an NLMSG_ERROR packet with the
193 error field set to 0. The application must generate acknowledgements
194 for received messages itself. The kernel tries to send an NLMSG_ERROR
195 message for every failed packet. A user process should follow this
196 convention too.
197 However, reliable transmissions from kernel to user are impossible in
199 any case. The kernel can't send a netlink message if the socket buffer
200 is full: the message will be dropped and the kernel and the user-space
201 process will no longer have the same view of kernel state. It is up to
202 the application to detect when this happens (via the ENOBUFS error re‐
203 turned by recvmsg(2)) and resynchronize.
204 Address formats
206 The sockaddr_nl structure describes a netlink client in user space or
207 in the kernel. A sockaddr_nl can be either unicast (only sent to one
208 peer) or sent to netlink multicast groups (nl_groups not equal 0).
209 struct sockaddr_nl {
211 sa_family_t nl_family; /* AF_NETLINK */
212 unsigned short nl_pad; /* Zero */
213 pid_t nl_pid; /* Port ID */
214 __u32 nl_groups; /* Multicast groups mask */
215 };
216 nl_pid is the unicast address of netlink socket. It's always 0 if the
218 destination is in the kernel. For a user-space process, nl_pid is usu‐
219 ally the PID of the process owning the destination socket. However,
220 nl_pid identifies a netlink socket, not a process. If a process owns
221 several netlink sockets, then nl_pid can be equal to the process ID
222 only for at most one socket. There are two ways to assign nl_pid to a
223 netlink socket. If the application sets nl_pid before calling bind(2),
224 then it is up to the application to make sure that nl_pid is unique.
225 If the application sets it to 0, the kernel takes care of assigning it.
226 The kernel assigns the process ID to the first netlink socket the
227 process opens and assigns a unique nl_pid to every netlink socket that
228 the process subsequently creates.
229 nl_groups is a bit mask with every bit representing a netlink group
231 number. Each netlink family has a set of 32 multicast groups. When
232 bind(2) is called on the socket, the nl_groups field in the sockaddr_nl
233 should be set to a bit mask of the groups which it wishes to listen to.
234 The default value for this field is zero which means that no multicasts
235 will be received. A socket may multicast messages to any of the multi‐
236 cast groups by setting nl_groups to a bit mask of the groups it wishes
237 to send to when it calls sendmsg(2) or does a connect(2). Only pro‐
238 cesses with an effective UID of 0 or the CAP_NET_ADMIN capability may
239 send or listen to a netlink multicast group. Since Linux 2.6.13, mes‐
240 sages can't be broadcast to multiple groups. Any replies to a message
241 received for a multicast group should be sent back to the sending PID
242 and the multicast group. Some Linux kernel subsystems may additionally
243 allow other users to send and/or receive messages. As at Linux 3.0,
244 the NETLINK_KOBJECT_UEVENT, NETLINK_GENERIC, NETLINK_ROUTE, and
245 NETLINK_SELINUX groups allow other users to receive messages. No
246 groups allow other users to send messages.
247 Socket options
249 To set or get a netlink socket option, call getsockopt(2) to read or
250 setsockopt(2) to write the option with the option level argument set to
251 SOL_NETLINK. Unless otherwise noted, optval is a pointer to an int.
252 NETLINK_PKTINFO (since Linux 2.6.14)
254 Enable nl_pktinfo control messages for received packets to get
255 the extended destination group number.
256 NETLINK_ADD_MEMBERSHIP, NETLINK_DROP_MEMBERSHIP (since Linux 2.6.14)
258 Join/leave a group specified by optval.
259 NETLINK_LIST_MEMBERSHIPS (since Linux 4.2)
261 Retrieve all groups a socket is a member of. optval is a
262 pointer to __u32 and optlen is the size of the array. The array
263 is filled with the full membership set of the socket, and the
264 required array size is returned in optlen.
265 NETLINK_BROADCAST_ERROR (since Linux 2.6.30)
267 When not set, netlink_broadcast() only reports ESRCH errors and
268 silently ignore ENOBUFS errors.
269 NETLINK_NO_ENOBUFS (since Linux 2.6.30)
271 This flag can be used by unicast and broadcast listeners to
272 avoid receiving ENOBUFS errors.
273 NETLINK_LISTEN_ALL_NSID (since Linux 4.2)
275 When set, this socket will receive netlink notifications from
276 all network namespaces that have an nsid assigned into the net‐
277 work namespace where the socket has been opened. The nsid is
278 sent to user space via an ancillary data.
279 NETLINK_CAP_ACK (since Linux 4.3)
281 The kernel may fail to allocate the necessary room for the ac‐
282 knowledgement message back to user space. This option trims off
283 the payload of the original netlink message. The netlink mes‐
284 sage header is still included, so the user can guess from the
285 sequence number which message triggered the acknowledgement.
286
288 The socket interface to netlink first appeared Linux 2.2.
289 Linux 2.0 supported a more primitive device-based netlink interface
291 (which is still available as a compatibility option). This obsolete
292 interface is not described here.
293
295 It is often better to use netlink via libnetlink or libnl than via the
296 low-level kernel interface.
297
299 This manual page is not complete.
300
302 The following example creates a NETLINK_ROUTE netlink socket which will
303 listen to the RTMGRP_LINK (network interface create/delete/up/down
304 events) and RTMGRP_IPV4_IFADDR (IPv4 addresses add/delete events) mul‐
305 ticast groups.
306 struct sockaddr_nl sa;
308 memset(&sa, 0, sizeof(sa));
310 sa.nl_family = AF_NETLINK;
311 sa.nl_groups = RTMGRP_LINK | RTMGRP_IPV4_IFADDR;
312 fd = socket(AF_NETLINK, SOCK_RAW, NETLINK_ROUTE);
314 bind(fd, (struct sockaddr *) &sa, sizeof(sa));
315 The next example demonstrates how to send a netlink message to the ker‐
317 nel (pid 0). Note that the application must take care of message se‐
318 quence numbers in order to reliably track acknowledgements.
319 struct nlmsghdr *nh; /* The nlmsghdr with payload to send */
321 struct sockaddr_nl sa;
322 struct iovec iov = { nh, nh->nlmsg_len };
323 struct msghdr msg;
324 msg = { &sa, sizeof(sa), &iov, 1, NULL, 0, 0 };
326 memset(&sa, 0, sizeof(sa));
327 sa.nl_family = AF_NETLINK;
328 nh->nlmsg_pid = 0;
329 nh->nlmsg_seq = ++sequence_number;
330 /* Request an ack from kernel by setting NLM_F_ACK */
331 nh->nlmsg_flags |= NLM_F_ACK;
332 sendmsg(fd, &msg, 0);
334 And the last example is about reading netlink message.
336 int len;
338 /* 8192 to avoid message truncation on platforms with
339 page size > 4096 */
340 struct nlmsghdr buf[8192/sizeof(struct nlmsghdr)];
341 struct iovec iov = { buf, sizeof(buf) };
342 struct sockaddr_nl sa;
343 struct msghdr msg;
344 struct nlmsghdr *nh;
345 msg = { &sa, sizeof(sa), &iov, 1, NULL, 0, 0 };
347 len = recvmsg(fd, &msg, 0);
348 for (nh = (struct nlmsghdr *) buf; NLMSG_OK (nh, len);
350 nh = NLMSG_NEXT (nh, len)) {
351 /* The end of multipart message */
352 if (nh->nlmsg_type == NLMSG_DONE)
353 return;
354 if (nh->nlmsg_type == NLMSG_ERROR)
356 /* Do some error handling */
357 ...
358 /* Continue with parsing payload */
360 ...
361 }
362
364 cmsg(3), netlink(3), capabilities(7), rtnetlink(7), sock_diag(7)
365 information about libnetlink ⟨ftp://ftp.inr.ac.ru/ip-routing/iproute2*⟩
367 information about libnl ⟨http://www.infradead.org/~tgr/libnl/⟩
369 RFC 3549 "Linux Netlink as an IP Services Protocol"
371
373 This page is part of release 5.12 of the Linux man-pages project. A
374 description of the project, information about reporting bugs, and the
375 latest version of this page, can be found at
376 https://www.kernel.org/doc/man-pages/.
377Linux 2021-03-22 NETLINK(7)